Scroll compressor differential pressure control during compressor shutdown transitions

ABSTRACT

A method including determining whether an OFF criterion is satisfied for a lag compressor of a tandem set of scroll compressors. The tandem set of scroll compressors includes a lead compressor. The method further includes: initiating at least one process when the OFF criterion is satisfied; and maintaining the lag compressor in an ON state for a predetermined period subsequent to the OFF criterion being satisfied. The at least one process includes at least one of: operating the lead compressor at a maximum level; overriding a motor overload protection method, wherein the motor overload protection method protects motors of the lead compressor and the lag compressor; and overriding a proportional integral derivative (PID) method to reduce a speed of a condenser fan. The PID method controls the speed of the condenser fan.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/765,884, filed on Feb. 18, 2013. The entire disclosure of the aboveapplication is incorporated herein by reference.

FIELD

The present disclosure relates to high-efficiency cooling systems, andmore particularly, to pressure control systems.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Cooling systems have applicability in a number of different applicationswhere a fluid is to be cooled. The fluid may be a gas, such as air, or aliquid, such as water. Example applications are heating, ventilation,air conditioning (HVAC) systems that are used for cooling spaces wherepeople are present such as offices and data center climate controlsystems. A data center may refer to a room having a collection ofelectronic equipment, such as computer servers.

In FIG. 1, an air conditioner 50 that may be used in, for example, acomputer room is shown. The air conditioner 50 includes a coolingcircuit 51 and a cabinet 52. The cooling circuit 51 is disposed in thecabinet 52 and includes an evaporator 54, an air moving device 56, acompressor 58, a condenser 60, and an expansion valve 62. The evaporator54, compressor 58, condenser 60 and expansion valve 62 are connected ina closed loop in which a cooling fluid (e.g., phase change refrigerant)circulates. The evaporator 54 may include a V-coil assembly withmultiple cooling slabs to provide increased cooling capacity. Theevaporator 54 receives the cooling fluid and cools air passing throughopenings in evaporator 54. The air moving device 56 (e.g., a fan orsquirrel cage blower) draws the air from an inlet (not shown) in thecabinet 52 and through the evaporator 54. The cooled air is directedfrom the evaporator 54 and out a plenum 64 in the cabinet 52.

The compressor 58 circulates the cooling fluid through the condenser 60,the expansion valve 62, the evaporator 54 and back to the compressor 58.The compressor 58 may be, for example, a scroll compressor. A scrollcompressor may be a fixed capacity or digital variable capacitycompressor. A scroll compressor typically includes two offset spiraldisks. The first spiral disk is a stationary disk or scroll. The secondspiral disk is an orbiting scroll. The cooling fluid is received at aninlet of the scroll compressor, trapped between the offset spiral disks,compressed, and discharged at a center (or outlet) towards the condenser60. The condenser 60 may be a micro-channel condenser that cools thecooling fluid received from the compressor 58. The expansion valve 62may be an electronic expansion valve and expand the cooling fluid out ofthe condenser 60 from, for example, a liquid to a vapor.

Although a single cooling circuit is shown in FIG. 1, multiple coolingcircuits may be included and arranged in series to provide multiplestages of cooling. The cooling circuits may minimize energy consumptionby reducing pressure ratios of the compressors. A pressure difference ofa compressor refers to a difference between an inlet or suction pressureand an outlet or head pressure of the compressor. To increase operatingefficiencies, the pressure differences may be further reduced based onestablished room conditions. The decrease in the pressure differencescan lead to the stationary scrolls of the compressors separating fromthe orbiting scrolls. This results in unpredictable and undirected“unloading” of the compressors. A compressor is unloaded when thecompressor is in a minimal vapor displacement condition (or transfers aminimal amount of fluid). The unloading of the compressors reduces thecooling capacity and temperature control stability of the coolingcircuits.

At elevated return air temperatures (90+° F./32.2+° C.) compressorefficiency gains can be maximized. At these temperatures and when, forexample, Refrigerant 410A is used and is at steady-state, operatingpressure can result in undirected unloading of the scroll compressors.

An air conditioning system may include one or more compressors includinga tandem set of compressors. The tandem set may include two or morecompressors of equal or unequal volumetric displacement. A firstcompressor may be a digital pulse width modulation (PWM) scrollcompressor that receives a PWM percentage signal to control capacity ofthe first compressor. As an alternative, the first compressor may be anyvariable capacity scroll compressor that receives a proportionalpercentage signal to control compressor speed. A second compressor maybe a fixed capacity scroll compressor with simply ON/OFF capacitycontrol. Additional compressors may be included in the tandem set. Theadditional compressors may be digital PWM scroll compressors, variablespeed scroll compressors, and/or fixed capacity scroll compressors.Suction and discharge lines of the compressors in the tandem set may bepiped in parallel to form the tandem set.

The tandem set offers an energy efficient configuration by allowing thedigital PWM scroll compressor to be activated prior to the fixedcapacity scroll compressor. This effectively allows the tandem set toprovide partial-displacement operation with a reduced volumetricdisplacement/capacity until additional capacity is needed from the fixedcapacity scroll compressor. While this partial-displacement operation isefficient, providing this operation in conjunction with a condenser atlow ambient temperatures and an evaporator at high ambient temperaturescan cause prolonged operation at a minimum compressor pressuredifference. This low-pressure difference can also cause an unloadedcompressor condition, which can lead to reduced cooling capacity of thecompressors.

The unloaded compressor condition at startup is prevalent at low-outdoorambient temperatures (e.g., temperatures less than 40° F./4.4° C.). Asan outdoor ambient temperature descends and an incoming air temperatureto the condenser is reduced, the duration of unloaded time may beincreased.

The compressor configuration of the tandem set also allows for energyefficient temperature control by providing a wide range of capacitymodulation for a cooling circuit of an air conditioning system. However,when an incoming load to the tandem set is reduced such that the fixedcapacity scroll compressor is deactivated, a rate of volumetricdisplacement for the tandem set can be reduced by at least 50%. This cancause the pressure difference of the tandem set to temporarily decreaseto a low-pressure difference. When the pressure difference is less thana predetermined pressure difference, the suction pressures and the headpressures of the compressors are approximately equal to each other,resulting in an unloaded condition. This reduces the ability of thetandem set to move vapor, which reduces cooling capacity.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

In an aspect, a method is provided and includes determining whether anOFF criterion is satisfied for a lag compressor of a tandem set ofscroll compressors. The tandem set of scroll compressors comprises alead compressor and the lag compressor. At least one process isinitiated when the OFF criterion is satisfied. The lag compressor ismaintained in an ON state for a predetermined period subsequent to theOFF criterion being satisfied. The at least one process includes atleast one of: (i) operating the lead compressor at a maximum level; (ii)overriding a motor overload protection method; and (iii) overriding aPID method to reduce a speed of a condenser fan. The motor overloadprotection method protects motors of the lead compressor and the lagcompressor. The PID method controls the speed of the condenser fan.

In another aspect, a method is provided and includes determining apressure difference across a scroll compressor of a cooling system. Thepressure difference is compared to a minimum differential pressurevalue. Pressure differences across the scroll compressor that are lessthan the minimum differential pressure value are associated withundirected unloading the scroll compressor. Parameters including a headpressure set point value and an absolute minimum pressure value areincreased when the pressure difference is less than the minimumdifferential pressure value. Subsequent to the increasing of theparameters, the parameters are decreased when the pressure difference isgreater than a sum of the minimum differential pressure value and ahysteresis value.

In another aspect, a method is provided and includes determining apressure difference across a scroll compressor of a cooling system. Thepressure difference is compared to a minimum differential pressurevalue. Pressure differences across the scroll compressor that are lessthan the minimum differential pressure value are associated withundirected unloading the scroll compressor. Parameters including anevaporator fan decrease value, an evaporator fan minimum value, acompressor increase value, and a compressor maximum value are decreasedwhen the pressure difference is less than the minimum differentialpressure value. Subsequent to the decreasing of the parameters, theparameters are increased when the pressure difference is greater than asum of the minimum differential pressure value and a hysteresis value

In another aspect, a method is provided and includes determining apressure difference across a scroll compressor of a cooling system. Thepressure difference is compared to a first minimum differential pressurevalue. Pressure differences across the scroll compressor that are lessthan the first minimum differential pressure value are associated withundirected unloading the scroll compressor. A first method is started toincrease first parameters when the pressure difference is less than thefirst minimum differential pressure value. The pressure difference iscompared to a second minimum differential pressure value. Pressuredifferences across the scroll compressor that are less than the secondminimum differential pressure value are associated with unloading thescroll compressor. A second method is started to decrease secondparameters when the pressure difference is less than the second minimumdifferential pressure value.

In another aspect, a method is provided and includes determining acooling value. The cooling value is compared to an activation point of alead compressor. The lead compressor is in a tandem set of scrollcompressors of a cooling system. The tandem set of compressors includesa lag compressor. The lead compressor is activated when the coolingvalue is greater than the activation point. The lag compressor isactivated subsequent to activating the lead compressor. The methodfurther includes determining whether conditions exist including: (i) analarm associated with the lag compressor is generated; and (ii) the leadcompressor is deactivated. The lag compressor is deactivated when atleast one of the conditions exists in the cooling system.

In another aspect, a method is provided and includes controlling acondenser fan at a first speed according to a proportional integralderivative (PID) method. The method further includes determining whethera scroll compressor in a tandem set of compressors in a cooling systemhas been activated. The PID method is overridden when the scrollcompressor has started including controlling the condenser fan at asecond speed based on an ambient temperature, a predeterminedtemperature and a first predetermined speed for the condenser fan. Thesecond speed is less than the first speed and greater than zero.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a perspective view of a prior art air conditioner;

FIG. 2 is a schematic view of a multi-stage cooling system incorporatinga cooling control module in accordance with an aspect of the presentdisclosure;

FIG. 3 is a functional block diagram of the cooling control module ofFIG. 2;

FIG. 4 is a functional block diagram of an operations control module ofFIG. 3;

FIGS. 5A and 5B provide a logic flow diagram illustrating a headpressure management method in accordance with an aspect of the presentdisclosure;

FIG. 6 is a pressure difference plot associated with the head pressuremanagement of FIGS. 5A and 5B;

FIGS. 7A and 7B provide a logic flow diagram illustrating a suctionpressure management method in accordance with an aspect of the presentdisclosure;

FIG. 8 is an evaporator fan speed and compressor duty cycle plotassociated with the suction pressure management of FIGS. 7A and 7B;

FIG. 9 is a compressor plot illustrating pulse width modulation dutycycle percentages in accordance with an aspect of the presentdisclosure;

FIG. 10 is a pressure difference plot associated with the suctionpressure management of FIGS. 7A and 7B;

FIG. 11 is a logic flow diagram illustrating a dual-pressure managementmethod with evaporator fan and compressor motor control in accordancewith an aspect of the present disclosure;

FIG. 12 is a functional block diagram of a startup control module inaccordance with an aspect of the present disclosure;

FIG. 13 is a logic flow diagram illustrating a lag compressor basedstartup pressure difference method in accordance with an aspect of thepresent disclosure;

FIG. 14 is a compressor status and cooling value plot associated withthe lag compressor based startup pressure difference method of FIG. 13;

FIG. 15 is a condenser fan based startup pressure difference method foruse with non-tandem compressors in accordance with an aspect of thepresent disclosure;

FIG. 16 is a condenser fan based startup pressure difference method foruse with tandem compressors in accordance with an aspect of the presentdisclosure;

FIG. 17 is a condenser fan speed interpolation-based plots for themethods of FIGS. 15 and 16;

FIG. 18 is a condenser fan speed plot for operation with tandemcompressors in accordance with an aspect of the present disclosure;

FIG. 19 is a functional block diagram of a shutdown control module inaccordance with an aspect of the present disclosure;

FIG. 20 is a logic flow diagram of a compressor transition pressuredifference method in accordance with an aspect of the presentdisclosure; and

FIG. 21 is a digital scroll plot for the method of FIG. 20;

FIG. 22 is an electronic expansion valve (EEV) plot for the method ofFIG. 20; and

FIG. 23 is a condenser fan speed plot for the method of FIG. 20.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

The following described embodiments prevent undirected unloading ofscroll compressors. The embodiments include techniques to ensure thateach pressure ratio or pressure difference of each scroll compressor ismaintained at or above a predetermined level. A pressure ratio refers toa ratio between input and output pressures of one or more compressors. Apressure difference refers to a difference between input and outputpressures of one or more compressors. The predetermined level may begreater than or equal to a level associated with undirected unloading anassociated scroll compressor. Although the following techniques areprimarily described with respect to pressure differences, pressureratios may be determined and used.

Techniques generally used during operation of tandem scroll compressorsare directed to FIGS. 2-11. Techniques generally used during startup ofthe scroll compressors are directed to FIGS. 2-3 and 12-18. Techniquesgenerally used during shutdown of the tandem scroll compressors aredirected to FIGS. 2-3 and 19-23.

In FIG. 2, a schematic of a cooling system 100 is shown. The coolingsystem 100 includes an upstream cooling stage 102 with an upstream (orfirst) cooling circuit 104 and a downstream (or second) cooling stage106 with a downstream cooling circuit 108. The cooling circuits arecontrolled via a cooling control module 109. The upstream coolingcircuit 104 includes a first evaporator 110, a first expansion valve112, a first condenser 114, a first compressor 116, and a secondcompressor 118. The downstream cooling circuit 108 includes a secondevaporator 120, a second expansion valve 122, a second condenser 124, athird compressor 126, and a fourth compressor 128. The evaporators 110,120 may have respective evaporator fans 130, 132. Although multipleevaporators and respective evaporator fans are shown, one or moreevaporators and one or more evaporator fans may be included. One or moreevaporator fans may be shared by one or more evaporators. The condensers114, 124 have respective condenser fans 134, 136.

The cooling control module 109 may generate condenser fan signals COND1.COND2, evaporator fan signals EVAP1, EVAP2, expansion valve signalsEXP1, EXP2, and compressor signals PWM1, PWM2, PWM3, PWM4 to control thefans 130, 132, 134, 136, expansion valves 112, 122, and the compressors116, 118, 126, 128.

The cooling control module 109 may control the fans 130, 132, 134, 136,the expansion valves 112, 122, and/or the compressors 116, 118, 126, 128based on signals from various sensors. The sensors may include, forexample, an ambient temperature sensor 150, suction pressure sensors152, 154, and/or head pressure sensors 156, 158. The ambient temperaturesensor 150 may be an outdoor ambient temperature sensor and generate anambient temperature signal T_(A). The pressure sensors 152, 154 generatesuction pressure signals SUC1, SUC2 and detect pressures of fluidreceived by the compressors 116, 118, 126, 128. The head pressuresensors 156, 158 generate head pressure signals HEAD1, HEAD2 and maydetect pressures of fluid out of the compressors 116, 118, 126, 128. Thehead pressure sensors 156, 158 may be located anywhere between thecompressors 116, 118, 126, 128 and the expansion valves 112, 122 anddetect pressures anywhere between the compressors 116, 118, 126, 128 andthe expansion valves 112, 122.

The evaporators 110, 120 may include, for example, micro-channel (MC)cooling coil assemblies, a MC heat exchanger, fin-and-tube cooling coilassemblies, and/or other evaporator components and assemblies. Theexpansion valves 112, 122 may be EEVs and/or thermostatic expansionvalves. Each of the condensers 114, 124 may be of a variety of types,such as an air-cooled condenser, a water-cooled condenser, or glycolcooled condenser. The condensers 114, 124 may include heat rejectiondevices that transfer heat from return fluids to a cooler medium, suchas outside air. The heat rejection devices may include air or liquidcooled heat exchangers.

In each of the circuits 104, 108, a cooling fluid (or refrigerant) iscirculated by a respective pair of the compressors 116, 118, 126, 128.The fluids flow from the compressors 116, 118, 126, 128, through thecondensers 114, 124, expansion valves 112, 122, and evaporators 110, 120and back to the compressors 116, 118, 126, 128. The evaporators 110, 120are arranged in stages such that air flows in a serial fashion firstthrough the upstream evaporator 110 and then through the downstreamevaporator 120. By having multiple cooling stages arranged for serialair flow, a temperature differential across the evaporators 110, 120 isreduced. This in turn allows the evaporators 110, 120 to operate atdifferent pressure levels and allows the pressure differences betweenthe respective evaporators 110, 120 and condensers 114, 124 to bereduced.

Since compressor electrical input power is a function of a pressuredifference between an evaporator and a condenser, a lower pressuredifference is more energy efficient. Each of the cooling circuits 104,108 may include a tandem set of compressors (e.g., compressors 116, 118or compressors 126, 128). Although the tandem set of compressors isshown as including two compressors, the tandem set of compressors mayinclude two or more compressors. Each of the tandem compressors may be afixed capacity scroll compressor (e.g., compressors 116, 126) or avariable capacity scroll compressor (e.g., compressors 118, 128). Thevariable capacity scroll compressors may be controlled via a respectivedigital signal received from the cooling control module 109.

Referring now also to FIG. 3, the cooling control module 109 is shown.The cooling control module 109 includes an operations control module160, a startup control module 162, and a shutdown control module 164.The operations control module 160 generally controls operation of thecooling circuits 104, 108 during operation of the compressors 116, 118,126, 128. The startup control module 162 generally controls operation ofthe cooling circuits 104, 108 during startup of the compressors 116,118, 126, 128. The shutdown control module 164 generally controlsoperation of the cooling circuits 104, 108 during shutdown of thecompressors 116, 118, 126, 128.

Referring now also to FIG. 4, the operations control module 160 isshown. The operations control module 160 performs a steady-statepressure difference (SSPD) method to control pressure differences acrossthe compressors 116, 118, 126, 128. If a pressure difference is lessthan a predetermined pressure (i.e. a low-pressure difference), theoperations control module 160 via the SSPD method adjusts controlparameters to ensure that an appropriate level of pressure difference ismaintained. The control parameters may be incrementally adjusted.

The SSPD method includes two distinct control methods; a head (high)pressure management method (referred to as SSPD-HP) illustrated in FIGS.5A-5B, and suction (low) pressure management method (referred to asSSPD-LP) illustrated in FIGS. 7A-7B. Both the SSPD-HP and SSPD-LPmethods may leverage functionality of related pre-existing controlmethods focusing solely on a head pressure and/or a suction pressure.Separate SSPD-HP methods may be executed for each of the coolingcircuits 104, 108. Separate SSPD-LP methods may be executed for each ofthe cooling circuits 104, 108. For the implementation of FIG. 2, twoSSPD-HP methods and two SSPD-LP methods may be executed in parallel. Inother words, each of the SSPD-HP, SSPD-LP methods are executed whileeach of the other SSPD-HP, SSPD-LP methods are executed.

The SSPD-HP method is eligible for use by a cooling circuit having anair-cooled condenser (e.g., one of the condensers 114, 124) withvariable speed fans (e.g., the condenser fans 134, 136), where the fansare controllable via a control module (e.g., the operations controlmodule 160). The SSPD-LP method is eligible for use by a cooling circuithaving a digital pulse width modulated (PWM) compressor (e.g., one ofthe compressors 116, 118, 126, 128) and/or a variable speed evaporatorfans (e.g., the evaporator fans 130, 132).

The operations control module 160 includes a first parameter module 170,a compressor pressure module 172, a first counter module 174, ahigh-pressure comparison module 176, a parameter increase module 178, aparameter decrease module 180, and a high-pressure management module 182with a condenser module 184. These modules are described with respect tothe following method of FIGS. 5A-5B.

Referring now also to FIGS. 5A-5B, in which a head pressure management(or SSPD-HP) method is shown. The head pressure management method maybegin at 200. The following tasks 202-224 are part of a first portion ofa SSPD-HP method. Subsequent tasks 226-234 are part of a second portionof the SSPD-HP method.

At 202, the first parameter module 170 determines various parametersincluding a high-pressure (HP)-X series of parameters and ahigh-pressure pressure difference (HPPD)-X series of parameters. TheHP-X series of parameters include a pressure set point value HP-A and anabsolute minimum pressure value HP-B. An absolute pressure is equal to ameasured pressure plus the atomic pressure or 14.7 pounds per squareinch (PSI). The units-of-measure for an absolute pressure is pounds persquare inch (PSIA). The HPPD-X series of parameters includes a minimumdifferential pressure value HPPD-A, a hysteresis value HPPD-B, anadjustment step value HPPD-C, an adjustment time value HPPD-D, a maximumset point value HPPD-E, and a maximum absolute pressure value HPPD-F.The values HP-A, HP-B may be stored in a memory 186 and accessed by theoperations control module 160. These values may have original versions(denoted with a subscript O) and/or updated versions (denoted with asubscript U).

The pressure set point value HP-A is a head pressure value maintainedusing a proportional integral derivative (PID) method to control acondenser fan (e.g., one of the fans 134, 136). The PID method may beimplemented via the condenser module 184, which generates a condensersignal COND to control the speed of the condenser fan. A PID method mayuse PID coefficients or gain values in a feedback algorithm to providean output (e.g., the condenser signal COND). The absolute minimumpressure value HP-B refers to a minimum head pressure below which theoperations control module 160 reduces speeds of the condenser fans to aminimum speed to prevent further reductions in head pressure.

The minimum differential value HPPD-A is a minimum amount of pressuredifference, which is maintained. The parameter HPPD-A may be associatedwith undirected compressor unloading. Compressor unloading may occur atpressure differences less than the parameter HPPD-A and/or at pressuredifferences less than a result of the parameter HPPD-A minus apredetermined safety margin. The hysteresis value HPPD-B is an amount ofpressure above the minimum differential value HPPD-A, which is attainedto begin incrementally reversing increases made during the SSPD-HPmethod. The adjustment step value HPPD-C is an amount of incrementaladjustment, which is applied to the parameters HP-A and HP-B when theSSPD-HP method is active.

The adjustment time value HPPD-D is an amount of time, which ispermitted to lapse before the adjustment step value HPPD-C is adjusted.The maximum set point value HPPD-E is a highest allowable value, whichthe pressure set point value HP-A can be set to using the SSPD-HPmethod. The maximum absolute pressure value HPPD-F is a highestallowable value, which the absolute minimum pressure value HP-B can beset to using the SPPD-HP method.

At 204, the compressor pressure module 172 determines the suctionpressure and the head pressure. The compressor pressure module 172 mayreceive a suction pressure signal SUC and a head pressure signal HEAD.

At 206, the compressor pressure module 172 determines a pressuredifference based on the pressure signals SUC, HEAD to generate apressure difference signal DIFF. The pressure difference signal DIFFmaybe calculated by subtracting the suction pressure signal SUC from thehead pressure signal HEAD.

At 208, the high-pressure comparison module 176 compares the pressuredifference signal DIFF to the minimum differential value HPPD-A andgenerates a first high-pressure comparison signal HCOM1. If the pressuredifference signal DIFF is less than the minimum differential valueHPPD-A, then task 210 is performed, otherwise task 209 may be performed.

At 210, the first counter module 174 resets or increments a firstcounter value to generate a first count signal CNT1 based on the firsthigh-pressure comparison signal HCOM1. The first counter value may bereset when a first iteration of tasks 208-212 is performed andincremented for subsequent iterations. At 212, the parameter increasemodule 178 determines whether the first count signal CNT1 is greaterthan the adjustment time value HPPD-D. If the first count signal CNT1 isgreater than the adjustment time value HPPD-D, then task 214 isperformed, otherwise task 204 may be performed.

At 209, the high-pressure comparison module 176 determines whether thepressure difference signal DIFF is greater than a sum of the valuesHPPD-A, HPPD-B and generates a second high-pressure comparison signalHCOM2. If the pressure difference signal DIFF is not greater than thesum, then task 211 is performed, otherwise task 226 is performed. At211, the first counter value is reset. Task 204 is performed subsequentto task 211.

At 214, the high-pressure comparison module 176 compares the pressureset point value HP-A to the maximum set point value HPPD-E and generatesa third high-pressure comparison signal HCOM3. Task 216 is performedwhen the pressure set point value HP-A is greater than or equal tomaximum set point value HPPD-E, otherwise task 218 is performed.

At 216, the parameter increase module 178 or other module of theoperations control module 160 sets the pressure set point value HP-Aequal to the maximum set point value HPPD-E.

At 218, the parameter increase module 178 increases the value HP-A. Thevalue HP-A may be incremented or increased by a predetermined amount(e.g., the amount indicated by the adjustment step value HPPD-C). TheSSPD-HP method adjusts the value HP-A to alter results of a PID controlmethod, which controls the head pressure. The PID control method mayadjust speeds of the condenser fans 134, 136 to adjust the headpressure. To control characteristics of how the value HP-A are adjustedby the SSPD-HP method, the parameters HPPD-A, HPPD-B, HPPD-C, HPPD-D,HPPD-E, HPPD-F are used. The parameters HPPD-A, HPPD-B, HPPD-C, HPPD-D,HPPD-E, HPPD-F may be shared for each of the cooling circuits 104, 108and separate SSPD-HP methods may be applied independently to each of thecooling circuits 104, 108.

At 220, the high-pressure comparison module 176 compares the absoluteminimum pressure value HP-B to the maximum absolute pressure valueHPPD-F and generates a fourth high-pressure comparison signal HCOM4. Ifthe absolute minimum pressure value HP-B is greater than or equal to themaximum absolute pressure value HPPD-F, then task 222 is performed,otherwise task 224 is performed.

At 222, the parameter increase module 178 or other module of theoperations control module 160 sets the absolute minimum pressure valueHP-B equal to the maximum absolute pressure value HPPD-F.

At 224, the parameter increase module 178 increases or increments theabsolute minimum pressure value HP-B. The absolute minimum pressurevalue HP-B may be incremented or increased by a predetermined amount(e.g., the amount indicated by the adjustment step value HPPD-C). TheSSPD-HP method adjusts the value HP-B to alter results of a PID controlmethod, which controls the head pressure. The PID control method mayadjust speeds of the condenser fans 134, 136 to adjust the headpressure. To control characteristics of how the value HP-B are adjustedby the SSPD-HP method, the parameters HPPD-A, HPPD-B, HPPD-C, HPPD-D,HPPD-E, HPPD-F are used. The parameters HPPD-A, HPPD-B, HPPD-C, HPPD-D,HPPD-E, HPPD-F may be shared for each of the cooling circuits 104, 108and separate SSPD-HP methods may be applied independently to each of thecooling circuits 104, 108.

At 226, the first counter value or the first count signal CNT1 is resetor incremented based on the first high-pressure comparison signal HCOM1.At 228, the parameter decrease module 180 determines whether the firstcount signal CNT1 is greater than the adjustment time value HPPD-D. Task230 is performed when the first count signal CNT1 is greater than theadjustment time value HPPD-D, otherwise task 204 is performed.

At 230, the parameter decrease module 180 may determine whether thevalues HP-A, HP-B (or updated values HP-A_(U), HP-B_(U)) are less thanor equal to their original values HP-A_(O), HP-B_(O) based on the firsthigh-pressure comparison signal HCOM1. If the values HP-A, HP-B are lessthan or equal to their original values, then task 232 is performed,otherwise task 234 is performed.

At 232, the parameter decrease module 180 or other module of theoperations control module 160 sets the values HP-A, HP-B to theiroriginal values HP-A_(O), HP-B_(O). At 234, the parameter decreasemodule 180 decreases or decrements the values HP-A, HP-B based on thesecond high-pressure comparison signal HCOM2. The parameter decreasemodule 180 may decrease each of the values HP-A, HP-B by a predeterminedamount (e.g., the amount indicated by the adjustment step value HPPD-C).Task 204 may be performed subsequent to tasks 232 and 234.

Referring now to FIG. 6, a pressure difference plot associated with thehead pressure management of FIGS. 5A and 5B is shown. FIG. 6 illustratesincremental increases and decreases in the values HP-A, HP-B andcorresponding changes in the pressure difference signal DIFF. FIG. 6includes line segments for the parameter HPPD-A and the sum of theparameters HPPD-A, HPPD-B.

As shown in FIG. 6, when the pressure difference signal DIFF decreasesbelow the minimum differential value HPPD-A for an amount of timedefined by the parameter HPPD-D, the values HP-A, HP-B are incremented,which increases the pressure difference signal DIFF. This causes speedsof the condenser fans 134, 136 to be reduced, which results in anincrease in head pressure. The increased head pressure results in anincrease in the pressure difference signal DIFF. The values HP-A, HP-Bare decremented when the pressure difference signal DIFF is greater thanthe sum of HPPD-A, HPPD-B for an amount of time defined by the parameterHPPD-D.

Referring again to FIG. 4, the operations control module 160 may furtherinclude a second parameter module 190, a second counter module 192, alow-pressure comparison module 194, and a low-pressure management module196 with an evaporator fan module 197 and a compressor module 198. Thesemodules are described with respect to the following suction pressuremanagement method of FIGS. 7A-7B. The suction pressure management (orSSPD-LP) method may begin at 250.

At 252, the second parameter module 190 determines various parametersincluding a low-pressure (LP)-X series of parameters and a low-pressurepressure difference (LPPD)-X series of parameters. The LP-X series ofparameters include an evaporator fan decrease value LP-A, an evaporatorfan minimum value LP-B, a compressor increase value LP-C, a compressormaximum value LP-D. The values LP-A, LP-B, LP-C, LP-D may be stored inthe memory 186 and accessed by the operations control module 160. Thesevalues may have original versions (denoted with a subscript O) and/orupdated versions (denoted with a subscript U).

The evaporator fan decrease value LP-A is a suction pressure at whichand/or above which speed of an evaporator fan is decreased to reduceevaporator pressure. The evaporator fan minimum value LP-B refers tosuction pressure at which or above which the speed of the evaporator fanis set to a minimum speed to reduce evaporator pressure. The compressorincrease value LP-C is a suction pressure at which or above which theoperations control module increases a pulse width modulated (PWM) dutycycle percentage (referred to hereinafter as PWM %) of a variablecapacity compressor to reduce evaporator pressure. The compressormaximum value LP-D is a suction pressure at which or above which theoperations control module operates the variable capacity compressor at amaximum PWM % to reduce evaporator pressure.

The SSPD-LP method adjusts a high-suction pressure management (HSPM)method by adjusting the LP-X series of parameters. The HSPM method isused to decrease speeds of an evaporator fan and increase a variablecompressor PWM % to reduce high evaporator pressures. The evaporator fanmodule 197 may generate an evaporator fan signal EVAP to adjust thespeed of the evaporator fan based on the values LP-A, LP-B. Thecompressor module 198 may generate a compressor signal PWM to controlthe variable compressor PWM % based on the suction pressure signal SUCand the values LP-C, LP-D.

Referring also to FIG. 8, an evaporator fan speed and compressor dutycycle plot associated with the HSPM method is shown. The plot of FIG. 8illustrates how adjustments in the LP-X series of parameters alters thespeed of the evaporator fan and the variable compressor PWM % to reducethe suction pressure signal SUC and/or prevent the suction pressuresignal SUC from exceeding a predetermined threshold.

If the suction pressure signal SUC is less than the evaporator fandecrease value LP-A, then the speed of the evaporator fan is not alteredbased on the HSPM method. The speed of the evaporator fan may bemaintained at a current speed via the evaporator fan module 197. If thesuction pressure signal SUC is between the values LP-A and LP-B, thenthe evaporator fan module 197 reduces the speed of the evaporator fan toa speed, which is determined by linearly interpolating between X and Yaxis positions associated with the values LP-A and LP-B. An interpolatedsegment 199 is shown between values LP-A, LP-B in FIG. 8. If the suctionpressure signal is greater than the evaporator fan minimum value LP-B,the evaporator fan module 197 adjusts the speed of the evaporator fan toa minimum evaporator fan speed. This decreases the fan speed asillustrated by the interpolated segment.

If the suction pressure signal SUC is less than the compressor increasevalue LP-C, then the compressor module 198 does not adjust the variablecompressor PWM % during the HSPM method. The variable compressor PWM %may be maintained at a current PWM %. If the suction pressure signal SUCis between the values LP-C, LP-D, then the compressor module increasesthe variable compressor PWM % to a level, which is determined bylinearly interpolating between X and Y axis positions of the valuesLP-C, LP-D. An interpolated segment 200 is shown between values LP-C,LP-D. If the suction pressure is greater than compressor maximum valueLP-D, the compressor module 198 operates the variable compressor at amaximum compressor PWM %.

The SSPD-LP method uses the LPPD-X series of parameters to influenceHSPM operation for the purpose of ensuring that a minimum compressorpressure difference is maintained. Although the LPPD-X series ofparameters may be shared between the cooling circuits 104, 108 of FIG.2, SSPD-LP methods may be independently executed for each of the coolingcircuits 104, 108. The LPPD-X series of parameters includes a minimumdifferential pressure value LPPD-A, a hysteresis value LPPD-B, anadjustment step value LPPD-C, an adjustment time value LPPD-D, and aminimum set point value LPPD-E.

The minimum differential pressure value LPPD-A is a minimum amount ofpressure difference, which is maintained. The parameter LPPD-A may beassociated with compressor unloading. Compressor unloading may occur atpressure differences less than LPPD-A and/or less than LPPD-A minus apredetermined safety margin. If the pressure difference drops below thevalue LPPD-A, a SSPD-LP method is activated, as illustrated by tasks252-278. The hysteresis value LPPD-B is an amount of pressure above theminimum differential pressure value LPPD-A, which is attained to beginincrementally reversing the decreases made during the SSPD-LP method.

The adjustment step value LPPD-C is an amount of incremental adjustment,which is applied to the values LP-A, LP-B, LP-C, LP-D when the SSPD-LPmethod is active. The adjustment time value LPPD-D is an amount of time,which lapses before another LPPD-C incremental adjustment can beapplied. The minimum set point value LPPD-E is a lowest allowable value,which the values LP-A, LP-C can be set to during the SSPD-LP method.

Referring again to FIGS. 7A-7B, at 254, the compressor pressure module172 determines the suction pressure and the head pressure. At 256, thecompressor pressure module 172 determines a pressure difference based onthe pressure signals SUC, HEAD to generate a pressure difference signalDIFF.

At 258, the low-pressure comparison module 194 compares the pressuredifference signal DIFF to the minimum differential value LPPD-A andgenerates a first low-pressure comparison signal LCOM1. If the pressuredifference signal DIFF is less than the minimum differential valueLPPD-A, then task 260 is performed, otherwise task 259 may be performed.

At 260, the second counter module 192 resets or increments a secondcounter value to generate a second count signal CNT2 based on the firstlow-pressure comparison signal LCOM1. The second counter value may bereset when a first iteration of tasks 258-262 is performed andincremented for subsequent iterations.

At 259, the low-pressure comparison module 194 determines whether thepressure difference signal DIFF is greater than a sum of the valuesLPPD-A, LPPD-B and generates a second low-pressure comparison signalLCOM2. If the pressure difference signal DIFF is not greater than thesum, then task 270 is performed, otherwise task 261 is performed. At261, the second counter value is reset. Task 254 is performed subsequentto task 261.

At 262, the parameter decrease module 180 determines whether the secondcount signal CNT2 is greater than the adjustment time value LPPD-D. Ifthe second count signal CNT2 is greater than the adjustment time valueLPPD-D, then task 264 is performed, otherwise task 254 may be performed.

At 264, the low-pressure comparison module 194 compares the pressure setpoint value LP-A to the minimum set point value LPPD-E and generates athird low-pressure comparison signal LCOM3. Task 254 is performed whenthe pressure set point value HP-A is less than or equal to minimum setpoint value LPPD-E, otherwise task 266 is performed.

At 266, the low-pressure comparison module 194 compares the compressorincrease value LP-C to the minimum absolute pressure value LPPD-E. Ifthe compressor increase value LP-C is less than or equal to the minimumabsolute pressure value LPPD-E, then task 254 is performed, otherwisetask 268 is performed.

At 268, the parameter decrease module 180 decreases the values LP-A,LP-B, LP-C, LP-D. The values LP-A, LP-B, LP-C, LP-D may be decrementedor decreased by a predetermined amount (e.g., the amount indicated bythe adjustment step value LPPD-C).

At 270, the second counter module 192 resets or increments the secondcount signal CNT2. At 272, the parameter decrease module 180 determineswhether the second count signal CNT2 is greater than the adjustment timevalue LPPD-D. Task 274 is performed when the second count signal CNT2 isgreater than the adjustment time value LPPD-D, otherwise task 254 isperformed.

At 274, the parameter decrease module 180 may determine whether thevalues LP-A, LP-B, LP-C, LP-D (or updated values LP-A_(U), LP-B_(U),LP-C_(U), LP-D_(U)) are greater than or equal to their original valuesLP-A_(O), LP-B_(O), LP-C_(O), LP-D_(O) based on the first low-pressurecomparison signal LCOM1. If the values LP-A, LP-B, LP-C, LP-D aregreater than or equal to their original values, task 276 is performed,otherwise task 278 may be performed.

At 276, the parameter increase module 178 set the values LP-A, LP-B,LP-C, LP-D to their original values LP-A_(O), LP-B_(O), LP-C_(O),LP-D_(O).

At 278, the parameter increase module 178 increases or increments thevalues LP-A, LP-B, LP-C, LP-D. The parameter increase module 178 mayincrease each of the values LP-A, LP-B, LP-C, LP-D by a predeterminedamount (e.g., the amount indicated by the adjustment step value LPPD-C).Task 254 may be performed subsequent to tasks 276 and 278.

In FIGS. 9-10, compressor plot and a pressure difference plot are shown.The compressor plot illustrates PWM percentages versus suction pressuresprovided during the method of FIGS. 7A-7B. The pressure difference plotillustrates compressor pressure differences during the method of FIGS.7A-7B. When the pressure difference signal DIFF decreases to a valueless than the value LPPD-A for the period defined by LPPD-D, the HSPMparameters or LP-A, LP-B, LP-C, LP-D are adjusted to increase thevariable compressor PWM %. The suction pressures to initiate the HSPMmethod are incrementally reduced by the SSPD-LP method when the pressuredifference signal DIFF is less than the value LPPD-A. This causes thespeed of the evaporator fan to be reduced and the variable compressorPWM % to be increased. These actions also cause a pressure of theevaporator to decrease, which results in an overall higher pressuredifference.

The methods of FIGS. 5A, 5B, 7A, 7B may both be performed to ensure thatan overall pressure difference(s) for compressor(s) are maintained abovea predetermined level to prevent undirected unloading of thecompressor(s). The predetermine level may be a manufacturer determinedlevel. The methods may be used to prevent unloading in, for example, airconditioning systems with scroll compressors, which operate withhigh-evaporating pressures and low-condensing pressures. An example ofcombining these methods is provided by the method of FIG. 11.

Referring now also to FIG. 11, a logic flow diagram illustrating adual-pressure management method is shown and illustrates SSPD-HP andSSPD-LP interaction. To prevent instances of system instabilityresulting from too many concurrent pressure-related adjustments, theoperations control module 160 may implement an interlock function. Theinterlock function ensures that the actions of the SSPD-HP method aresequenced appropriately with the actions of the SSPD-LP method. Thedual-pressure management method may begin at 300.

At 301, a compressor pressure difference is determined. At 302, thehigh-pressure comparison module 176 determines whether the pressuredifference is less than the minimum differential value HPPD-A. Task 304is performed when the pressure difference is less than the minimumdifferential value HPPD-A, otherwise task 308 may be performed.

At 304, the operations control module 160 starts the increase routine ofthe SSPD-HP method of FIG. 5A. Task 210 may be performed subsequent totask 304. The operations control module 160 may return to task 308 afterperforming task 224.

At 308, the low-pressure comparison module 194 determines whether thepressure difference signal DIFF is less than the minimum differentialvalue LPPD-A. Task 310 is performed when the pressure difference signalDIFF is less than the minimum differential value LPPD-A, otherwise task312 may be performed.

At 310, the operations control module 160 starts the decrease routine ofthe SSPD-LP method. Task 260 may be performed subsequent to task 310.The operations control module 160 may return to task 301 afterperforming task 268.

At 312, the low-pressure comparison module 194 determines whether thepressure difference signal DIFF is greater than a sum of the valuesLPPD-A and LPPD-B. Task 314 is performed when the pressure differencesignal DIFF is greater than the sum of the values LPPD-A and LPPD-B,otherwise task 316 is performed.

At 314, the operations control module 160 starts the decrease routine ofthe SSPD-HP method. Task 226 may be performed subsequent to task 314.The operations control module 160 may return to task 316 afterperforming task 232 and/or 234.

At 316, the low-pressure comparison module 194 determines whether thepressure difference signal DIFF is greater than a sum of the valuesLPPD-A and LPPD-B. Task 318 is performed when the pressure differencesignal DIFF is greater than the sum, otherwise task 301 is performed. At318, the parameter increase module 178 may set or increase the HSPMparameters and/or the values LP-A, LP-B, LP-C, LP-D by performing theincrease routine of the SSPD-LP method at task 270. The HSPM parametersand/or the values LP-A, LP-B, LP-C, LP-D may be increased to reversepreviously performed reductions. The operations control module 160 mayreturn to task 301 after performing task 276 and/or 278.

If one of the methods SSPD-HP or SSPD-LP are disabled (or not active),the other one of the methods may be enabled (or active) withoutdependence on the disabled method.

As an example, the methods of FIGS. 5A, 5B, 7A, 7B, and 11 may be usedto monitor and regulate compressor pressure difference(s) for an airconditioning system with tandem digital scroll compressor(s),condenser(s), and EEVs. As another example, the methods of FIGS. 5A, 5B,7A, 7B, and 11 may be used to monitor and regulate compressor pressuredifference(s) for an air conditioning system with fixed, digital, ortandem scroll compressor(s), a water-cooled condenser(s), and EEVs orthermostatic expansion valve(s).

The methods of FIGS. 5A, 5B, 7A, 7B, and 11 prevent undirected unloadingof compressors. As the methods are performed via the cooling controlmodule 109, the cooling system 100 and methods can be monitored,configured, enabled, and/or disabled via a user interface 329. The userinterface 329 may include a keypad, a keyboard, a mouse, a touch screen,or other suitable user interface.

In FIG. 12, the startup control module 162 is shown. The startup controlmodule 162 includes a temperature error module 330, a startup coolingmodule 332, a first startup comparison module 334, a delay timer 336, atemperature module 338, a second startup comparison module 340, a thirdstartup comparison module 342, a lead compressor module 344, a lagcompressor module 345, and a lag timer 349. The parameters used and/orgenerated by the modules of the startup control module 162 may be storedin the memory 186. Operation of these modules is described with respectto the method of FIG. 13.

Referring also to FIG. 13, a lag compressor based startup pressuredifference (SUPD) method is shown. The SUPD method may be performed foreach of the cooling circuits 104, 108 of FIG. 2. Each of the SUPDmethods includes two control methods. Each of the control methodsincludes operations independent of the other control method. The firstcontrol method includes starting both a lead compressor and a lagcompressor in a tandem set. The first control method may be referred toas the SUPD-lag compressor (LC) method.

The lead compressor is the compressor, which starts first. The lagcompressor has an activation point which is higher than the activationpoint of the lead compressor. Thus, startup of the lag compressorgenerally occurs after the startup of the lead compressor. The leadcompressor may start before the lag compressor, for example, when theSUPD-LC method is inactive. By default, a digital variable capacityscroll compressor may be the lead compressor and a fixed capacity scrollcompressor may be the lag compressor. Both of the lead and lagcompressors are started when cooling for a current operating conditioncan be satisfied by a cooling capacity of one of the lead and lagcompressors.

The second control method includes operating a condenser fan at a speedless than a previous or predetermined speed when the lead and lagcompressors start. The second control method is referred to as theSUPD-condenser fan (CF) method. The SUPD-LC method is available for anair conditioning system that has tandem variable capacity scrollcompressors and an outdoor ambient temperature sensor input (or asuction pressure input). The SUPD-CF method is available for airconditioning systems, which have an outdoor ambient temperature sensorinput and an air-cooled condenser. The air-cooled condenser has acondenser fan speed that can be controlled by a control module and thecontrol module activates the tandem variable capacity scrollcompressors. The tandem variable capacity scroll compressors areactivated based on a call for cooling value CFC for both of the SUPD-LCand SUPD-CF methods. The call for cooling value CFC is used primarily toactivate and deactivate compressors.

The SUPD-LC method includes activating the lag compressor after the leadcompressor, when a low-pressure difference across a compressor isdetected. This allows the tandem set to operate at a full rate ofvolumetric displacement to more rapidly increase the pressuredifference. The following parameters LCPD-A, LCPD-B, LCPD-C, LCPD-D,LCPD-E, LCPD-F, LCPD-X are used during the SUPD-LC method. The parameterLCPD-A is an outdoor ambient temperature associated with activating thelag compressor. The SUPD-LC method is started when the LCPD-Atemperature is reached.

The parameter LCPD-B refers to a lead compressor activation point. Whenthe call for cooling value CFC is greater than the parameter LCPD-B, thelead compressor is activated. The parameter LCPD-C refers to a leadcompressor deactivation point. When the call for cooling value CFC isless than the parameter LCPD-C, the lead compressor is deactivated. Theparameter LCPD-D refers to a lag compressor activation point. When thecall for cooling value CFC is greater than the parameter LCPD-D, the lagcompressor is activated. The parameter LCPD-E refers to a lag compressordeactivation point. When the call for cooling value CFC is less than theparameter LCPD-E, the lag compressor is deactivated.

The parameter LCPD-F refers to a compressor minimum ON time. This is anamount of time that each of the compressors is maintained in an ON oractive state once the compressor is started before deactivation of thecompressor. The parameter LCPD-X refers to a suction pressure abovewhich the lag compressor is activated. The SUPD-LC method may be startedwhen the suction pressure is greater than the parameter LCPD-X. Thesuction pressure may be used to activate the SUPD-LC method and/or toactivate the lag compressor when an outdoor ambient temperature readingis unavailable.

The lag compressor based SUPD method may begin at 350. At 352, thetemperature error module 330 determines a control temperature errorvalue CFE based on the ambient temperature T_(A) and a predeterminedtemperature setpoint SET. The control temperature error value CFE may beequal to the ambient temperature T_(A) minus the predeterminedtemperature setpoint SET.

At 354, the startup cooling module 332 determines the call for coolingvalue CFC based on or as a function of the control temperature errorvalue CFE. At 356, the first startup comparison module 334 compares thecall for cooling value CFC with the parameter LCPD-B to generate a firstCFC comparison signal SCOM1 _(B). If the call for cooling value CFC isgreater than the parameter LCPD-B, then task 358 is performed, otherwisetask 352 is performed.

At 358, the lead compressor module 344 generates a lead compressorsignal PWM based on the first comparison signal SCOM1. The leadcompressor is started first in response to the call for cooling valueCFC rising above a corresponding activation point.

At 360, the delay timer starts a first timer and generates a first delaytimer signal TIM1. At 361, the first startup comparison module 334compares the call for cooling value CFC with the parameter LCPD-D togenerate a second CFC comparison signal SCOM1 _(D). If the call forcooling value CFC is greater than the parameter LCPD-D, then task 370 isperformed, otherwise task 362 is performed.

At 362, the temperature module 338 determined the outdoor ambienttemperature T_(A) and/or the second startup comparison module 340determines the suction pressure, which may be based on the suctionpressure signal SUC. At 364, the temperature module 338 compares theoutdoor ambient temperature T_(A) with the parameter LCPD-A to generatea second comparison signal SCOM2. The second startup comparison module340 may alternatively or in addition compare the suction pressure signalSUC with the parameter LCPD-X to generate a third comparison signalSCOM3. When the outdoor ambient temperature T_(A) is less than theparameter LCPD-A and/or the suction pressure signal SUC is less than theparameter LCPD-X, task 366 is performed, otherwise task 368 isperformed.

If the startup control module 162 loses communication with the outdoorambient temperature sensor 150 or the cooling system 100 is not equippedwith an outdoor ambient temperature sensor, then the SUPD-LC method canbe activated based on the suction pressure signal SUC. The activationmay occur when the suction pressure signal SUC is less than theparameter LCPD-X and when the lead compressor is activated.

At 366, the lag compressor module 345 starts the SUPD-LC method based onthe second and third comparison signals SCOM2, SCOM3. Task 370 isperformed subsequent to task 366.

At 368, the lag compressor module 345 determines whether the first timersignal TIM1 is greater than a predetermine amount of time (e.g., 10seconds). If the result of task 368 is TRUE, the method ends at 369.

At 370, the lag compressor module 345 starts the lag (fixed) compressor.At 371, the lag timer 349 is started when the lag compressor isactivated to record an amount of time that the lag compressor is ONbased on the second and third comparison signals SCOM2, SCOM3. The ONtime of the lag compressor is indicated via a lag time signal TIMLAG.

At 372, the lag compressor module 345 determines whether an alarm signalALARM has been generated for the lag compressor. The alarm signal ALARMmay be generated, for example, when there is fault associated with theoperation of the lag compressor. Task 375 is performed when the resultof task 372 is TRUE, otherwise task 374 is performed. At 374, the lagcompressor module 345 determines whether the lead compressor has beenturned OFF based on the PWM signal. Task 375 is performed when the leadcompressor has been turned off, otherwise task 376 is performed. At 375,the lag compressor module 345 turns off the lag compressor. The leadcompressor may also be turned off, if the lead compressor is not alreadyturned off.

At 376, the lag compressor module 345 determines whether the lag timesignal TIMLAG is greater than the parameter LCPD-F. If the result oftask 376 is TRUE, task 368 is performed, otherwise task 372 isperformed.

In FIG. 14, a compressor status and cooling value plot associated withthe lag compressor based startup pressure difference method of FIG. 13is shown. FIG. 14 includes the outdoor ambient temperature signal T_(A),a call for cooling value signal CFC, and lead and lag compressor motorstatus signals. The lead and lag compressor motor status signalsindicate when the lead and lag compressors are activated anddeactivated. The parameters LCPD-A, LCPD-B, LCPD-C, LCPD-D, LCPD-E arealso shown as respective lines. FIG. 14 illustrates the lag compressorbeing activated shortly after the lead compressor is activated, althoughthe call for cooling value signal CFC is not greater than the activationthreshold identified by the parameter LCPD-D. The lag compressor isturned OFF when the call for cooling value signal CFC decreases to avalue less than the deactivation setpoint (identified by the parameterLCPD-E) and the minimum ON time (identified by the parameter LCPD-F) haslapsed.

Referring again to FIG. 12, the startup control module 162 may furtherinclude a condenser fan module 346. The condenser fan module 346 mayinclude a first fan control module 347 and a second fan control module348.

The condenser fan module 346 may execute a first startup pressuredifference method when non-tandem compressors are used and a secondstartup pressure difference method when tandem compressors are used. Themethods may be executed based on the outdoor ambient temperature T_(A),a parameter CFPD-A, parameters CFPD-TX, parameters CFPD-CAX, andparameters CFPD-CBX.

The parameter CFPD-A refers to a SUPD-CF active time and defines anamount of time in which the SUPD-CF method is to be active after thecompressor or tandem set is started. The parameters CFPD-TX refers toparameters CFPD-T1 through CFPD-T8, which are outdoor air temperaturesassociated with condenser fan speeds indicated by the parametersCFPD-CAX and CFPD-CBX.

The parameters CFPD-CAX refers to parameters CFPD-CA1 through CFPD-CA8,which are condenser fan speeds. The condenser fan speeds may be used ateach of the corresponding CFPD-TX temperatures when non-tandemcompressors are used. The parameters CFPD-CAX may also be used for alead compressor in a tandem set.

The parameters CFPD-CBX refer to parameters CFPD-CB1 through CFPD-CB8,which are condenser fan speeds. The condenser fan speeds may be used ateach of the corresponding CFPD-TX temperatures when the lag compressorin a tandem set is started before the timer associated with theparameter CFPD-A lapses. The parameters CFPD-CBX may be used with tandemscroll compressors. For air conditioning systems with multiple coolingcircuits, separate sets of parameters CFPD-CAX and CFPD-CBX are providedfor each individual cooling circuit.

Table 1 provides an example of parameters CFPD-TX, CFPD-CAX, andCFPD-CBX stored in tabular form.

TABLE 1 SUPD-CF Temperature and Fan Speed Parameters CFPD-T8 (HighestTemperature) CFPD-CA8 CFPD-CB8 CFPD-T7 CFPD-CA7 CFPD-CB7 CFPD-T6CFPD-CA6 CFPD-CB6 CFPD-T5 CFPD-CA5 CFPD-CB5 CFPD-T4 CFPD-CA4 CFPD-CB4CFPD-T3 CFPD-CA3 CFPD-CB3 CFPD-T2 CFPD-CA2 CFPD-CB2 CFPD-T1 (LowestTemperature) CFPD-CA1 CFPD-CB1

Referring also to FIG. 15, a condenser fan based startup pressuredifference method for use with each set of non-tandem compressors isshown. The method may begin at 400.

At 402, the first fan control module 347 controls speeds of thecondenser fans according to a condenser fan proportional integralderivative (CFPID) method. The CFPID method is performed to maintain adesired head pressure. The CFPID method is used to generate a condenserfan control signal CF, which controls the speeds of the condenser fans.

At 404, the condenser fan module 346 determines whether a compressor(e.g., the lead compressor and/or the lag compressor) has started basedon the lead compressor signal PWM and the lag compressor signal LAG. At406, the condenser fan module 346 or the second fan control module 348starts the SUPD-CF method and temporarily overrides the CFPID methodwhen one of the compressors has started. By doing this, the condenserfans are maintained at a reduced speed (e.g., a speed slower than whenthe CFPID method was active). The reduced condenser fan speedfacilitates rapid generation and increase in the head pressure, whichcontributes to a rapid establishment and increase in the pressuredifference during compressor startup.

At 408, the delay timer 336 starts a second timer and generates a secondtimer signal TIM2 based on the first comparison signal SCOM1, the leadcompressor signal PWM, and/or the lag compressor signal LAG. At 410, thecondenser fan module 346 and/or the second fan control module 348maintains the speeds of the condenser fans based on the outdoor ambienttemperature T_(A) and parameters CFPD-TX and CFPD-CAX. The condenser fanmodule 346 and/or the second fan control module 348 may interpolatebetween CFPD-TX and CFPD-CAX points to determine condenser fan speedsbased on the outdoor ambient temperature T_(A).

As Table 1 shows, each CFPD-CAX fan speed parameter is paired with itscorresponding CFPD-TX temperature. These pairings are then used tointerpolate the condenser fan speed to be used when the SUPD-CF methodis active, as shown in FIG. 17. FIG. 17 shows the condenser fan speedsthat may be used for the SUPD-CF method. The condenser fan speeds may bedetermined by linear interpolation between the appropriate CFPD-CAX andCFPD-TX points.

At 412, the condenser fan module 346 may determine whether the secondtimer signal TIM2 has exceeded a predetermined time or the timeindicated by the parameter CFPD-A. Task 414 is performed when the resultof task 412 is TRUE, otherwise task 410 may be performed to update thecondenser fan speed and the parameters T_(A), CFPD-TX, CFPD-CAX.

At 414, the condenser fan module 346 returns to controlling and/orramping speeds of the condenser fans according to the CFPID method. TheSUPD-CF method is deactivated. The second fan control module 348relinquishes control of the condenser fans to the first fan controlmodule 347. The method may end after task 414 at 416.

In FIG. 16, a condenser fan based startup pressure difference method foruse with each set of tandem compressors is shown. The method may beginat 450.

At 452, the first fan control module 347 controls speeds of thecondenser fans according to the CFPID method. The CFPID method isperformed to maintain a selected or predetermined head pressure. TheCFPID method is used to generate the condenser fan control signal CF,which controls the speeds of the condenser fans.

At 454, the condenser fan module 346 determines whether the leadcompressor has started based on the lead compressor signal PWM. At 406,the condenser fan module 346 or the second fan control module 348 startsthe SUPD-CF method and temporarily overrides the CFPID method when oneof the compressors has started. By doing this, the condenser fans aremaintained at a reduced speed (e.g., a speed slower than when the CFPIDmethod was active). The reduced condenser fan speed facilitates rapidgeneration and increase in the head pressure, which contributes to arapid establishment and increase in the pressure difference duringcompressor startup.

At 458, the delay timer 336 starts the second timer and generates thesecond timer signal TIM2. At 460, the condenser fan module 346 and/orthe second fan control module 348 maintains the speeds of the condenserfans based on the outdoor ambient temperature T_(A) and parametersCFPD-TX and CFPD-CAX. The condenser fan module 346 and/or the second fancontrol module 348 may interpolate between CFPD-TX and CFPD-CAX pointsto determine condenser fan speeds based on the outdoor ambienttemperature T_(A).

As Table 1 shows, each CFPD-CAX fan speed parameter is paired with itscorresponding CFPD-TX temperature. These pairings are then used tointerpolate the condenser fan speed to be used when the SUPD-CF methodis active, as shown in FIG. 17. FIG. 17 shows the condenser fan speedsthat may be used for the SUPD-CF method. The condenser fan speeds may bedetermined by linear interpolation between the appropriate CFPD-CAX andCFPD-TX points.

At 462, the condenser fan module 346 may determine whether the secondtimer signal TIM2 has exceeded a predetermined time or the timeindicated by the parameter CFPD-A. Task 464 is performed when the resultof task 462 is FALSE. Task 468 is performed when the result of task 462is TRUE. At 464, the second fan control module 348 determines whetherthe lag compressor has started based on the lag compressor signal LAG.If the lag compressor has started task 466 is performed, otherwise task460 may be performed to update the condenser fan speed and theparameters T_(A), CFPD-TX, CFPD-CAX.

At 466, the condenser fan module 346 and/or the second fan controlmodule 348 maintains the speeds of the condenser fans based on theoutdoor ambient temperature T_(A) and parameters CFPD-TX and CFPD-CBX.The condenser fan module 346 and/or the second fan control module 348may interpolate between CFPD-TX and CFPD-CBX points to determinecondenser fan speeds based on the outdoor ambient temperature T_(A).

As Table 1 shows, each CFPD-CBX fan speed parameter is paired with itscorresponding CFPD-TX temperature. These pairings are then used tointerpolate the condenser fan speeds. The condenser fan speeds may bedetermined by linear interpolation between the appropriate CFPD-CBX andCFPD-TX points.

At 467, the condenser fan module 346 may determine whether the secondtimer signal TIM2 has exceeded a predetermined time or the timeindicated by the parameter CFPD-A. Task 468 is performed when the resultof task 467 is TRUE, otherwise task 466 may be performed to update thecondenser fan speed and the parameters T_(A), CFPD-TX, CFPD-CBX.

At 468, the condenser fan module 346 returns to controlling and/orramping speeds of the condenser fans according to the CFPID method. TheSUPD-CF method is deactivated. The second fan control module 348relinquishes control of the condenser fans to the first fan controlmodule 347. The method may end after task 468 at 470.

In FIG. 18, a condenser fan speed plot for operation with tandemcompressors is shown. FIG. 18 shows an example of SUPD-CF operation fortandem compressors with the SUPD-LC method disabled (i.e. not being usedwhen the SUPD-CF method is being used).

The SUPD-CF method holds the condenser fan at a reduced speed duringstartup of the lead compressor, which causes the head pressure to riseabove the condenser pressure set point indicated by the rising CFsignal. When the lag compressor is activated, the condenser fan speed isincreased to avoid allowing the head pressure to climb high enough toactuate a high-pressure discharge switch which may reside in ahigh-pressure discharge line. The high-pressure discharge line may beconnected to, for example, a line 480 extending between the condenser114 and the evaporator 110 or a line 482 extending between the condenser124 and the evaporator 120 of FIG. 2. The parameter CFPD-A may be setto, for example, 5 minutes. As a result, the second timer may expire atthe 5 minute interval. Subsequent to the second timer expiring, thespeed of the condenser fan may be ramped towards the speed provided bythe CFPID method.

The methods of FIGS. 13, 15, and 16 may be used to manipulate compressorand condenser fan operation to provide a cooling capacity and/or outputintensity that is inversely proportionate to changes in an outdoorambient temperature. The cooling capacity is provided quickly. Thisprovides temperature stability and compressor reliability duringcompressor startup operation to prevent undirected unloading ofcompressors.

The parameters of the methods of FIGS. 13, 15, and 16 are configurablevia the user interface 329 of FIG. 2. The methods manipulate compressorand condenser fan operation based on an outdoor ambient temperature.This control proactively prevents low-compressor pressure differencesduring compressor startup and minimizes undirected compressor unloadingassociated with operating a tandem set of compressors in an unloadedcondition during compressor startup.

The methods of FIGS. 13, 15, and 16 may be applied to, for example, anair conditioning system with tandem digital scroll compressors, acondenser, and an EEV. As another example, the methods may be applied toan air conditioning system with fixed, digital, or tandem digital scrollcompressors, a water-cooled condenser, and an EEV or thermostaticexpansion valve.

In FIG. 19, the shutdown control module is shown. The shutdown controlmodule 164 may include the lag compressor module 345, a load durationtimer 484, an overwrite timer 485, a routine module 486, a leadcompressor module 487, an expansion valve module 488, a shutdownparameter module 489, and a condenser fan module 346. The parametersused and/or generated by the modules of the shutdown control module 164may be stored in the memory 186. The operations of the modules aredescribed with respect to the method of FIG. 20.

In FIG. 20, a compressor transition pressure difference (CTPD) method isshown. The CTPD method may be used for each tandem set of lead and lagcompressors. The CTPD method includes a digital scroll (DS) method, anelectronic expansion valve (EEV) motor overload protection (MOP) controlmethod, and a condenser fan reduction (FR) method, referred torespectively as the CTPD-DS, CTPD-EV, and CTPD-FR methods.

The CTPD-DS, CTPD-EV, and CTPD-FR methods are activated when a lagcompressor is being deactivated to satisfy the call for cooling valueCFC. The call for cooling value CFC may be used to modulate a compressorcapacity, turn compressors ON and OFF, and to determine a digital scrollcompressor PWM %. For the CTPD method, the relationship between the callfor cooling value CFC and the digital scroll compressor PWM % may beused to obtain a PWMCFC signal. The PWMCFC signal is the digital scrollcompressor PWM % determined based on the call for cooling value CFC. ThePWMCFC signal is a function of both the call for cooling value CFC and anumber of compressors which are ON. This dependence on the number ofactive compressors accounts for a discontinuity in the PWMCFC signalwhen the lag compressor is deactivated, which is shown by segment 490 inthe plot of FIG. 21.

For the CTPD method, the lead compressor may start before the lagcompressor when the call for cooling value CFC increases above acorresponding first activation point. The lag compressor has a secondactivation point, which is higher than the first activation point. As aresult, startup of the lag compressor occurs after startup of the leadcompressor. By default, the lead compressor may be a digital scrollcompressor and the lag compressor may be a fixed scroll compressor.

The CTPD method may begin at 500. At 501, the expansion valve module 488starts a motor overload protection (MOP) method to protect compressormotors via an overload module. This effectively reduces an openingpercentage of the EEV when the lag compressor in the tandem set turnsOFF. The expansion valve module 488 includes an overload module 491, anevaporator temperature module 492, an overwrite module 493, and anopening module 494.

An EEV may be used on each cooling circuit of a cooling system, as shownin FIG. 2. The evaporator temperature module 492 may control openingpercentages for the EEVs by executing an expansion valve proportionalintegral derivative (EVPID) method. The EVPID method is used to controlthe opening percentages to increase an evaporator temperature to asuperheat set point (i.e. a temperature greater than a predeterminedtemperature). The EVPID method may generate an EVPID value, which may bea valve opening percentage to attain a superheat condition, where theevaporator temperature is equal to the superheat set point. The openingmodule 494 may adjust the opening percentages of the EEVs based on theEVPID value.

The MOP method may include use of a parameter MOP-A, which refers to atemperature threshold. The temperature threshold is a temperature abovewhich the MOP method is initiated by the overload module 491. The MOPmethod is initiated if a saturated suction temperature T_(SUC) isgreater than the temperature threshold as calculated by the overloadmodule.

The purpose of the MOP method is to protect the compressor motor(s) fromincoming cooling fluid vapor pressures and/or temperatures that are toohigh (i.e. greater than a predetermined pressure and a predeterminedtemperature). When the saturated suction temperature T_(SUC) increasesabove the parameter MOP-A, the MOP method is started, the EVPID value isoverridden or not used, and the opening percentage of the EEV isreduced. The rate of this reduction is dependent on how far thesaturated suction temperature T_(SUC) is above the parameter MOP-A. Themore the saturated suction temperature T_(SUC) is above the parameterMOP-A, the faster the EEV is closed.

For any given cooling fluid or refrigerant, the saturated suctiontemperature T_(SUC) is purely a function of the suction pressure SUC.Reducing the suction pressure SUC, reduces the saturated suctiontemperature T_(SUC). When the MOP method reduces the opening percentageof the EEV, the head pressure HEAD increases and the suction pressureSUC decreases. While a corresponding increase in a compression ratio(e.g., head pressure HEAD divided by suction pressure SUC) is providedfor the MOP method, the increase in the compression ratio can also beprovided to increase or maintain the pressure difference (e.g., headpressure minus suction pressure).

The CTPD-EV method alters the parameter MOP-A to start the MOP method topartially close the EEV. By utilizing the MOP method to establish ahigher compression ratio before the lag compressor is turned OFF, it isless likely that pressure difference issues will arise. The CTPD-EVmethod is configured using parameters EVPD-A, EVPD-B, EVPD-C.

The parameter EVPD-A refers to a MOP temperature and defines a valueabove which the MOP-A temperature is set. The MOP-A temperature is setequal to the parameter EVPD-A when the CTPD-EV method is active. Theparameter EVPD-B refers to a CTPD-EV duration, which defines an amountof time that the parameter MOP-A is to be overwritten or replaced withthe parameter EVPD-A. The parameter EVPD-C refers to a MOP temperaturerestore rate which defines a rate at which the MOP-A temperature isrestored to an original value (the MOP-A is no longer overridden) oncethe CTPD-EV method becomes inactive.

At 502, the lag compressor module 345 determines whether a lagcompressor OFF criterion is satisfied. The lag compressor generates acriterion status signal CRIT to indicate when the OFF criterion issatisfied. An example criterion for turning OFF the lag compressor isprovided by the method of FIG. 13, which includes turning OFF the lagcompressor at 378, when at least one of the conditions at 372-376 issatisfied.

At 504, the lead compressor module 487, the expansion valve module 488,and the condenser fan module 346 initiate the CTPD-DS, CTPD-EV, andCTPD-FR methods based on the criterion status signal CRIT, as indicatedby respective signals DS, EV, FR. Tasks 506, 510, 517, and 528 may beperformed in parallel subsequent to task 504. The CTPD-DS methodincludes tasks 510-516. The CTPD-EV method includes tasks 517-526. TheCTPD-FR method includes tasks 528-530.

At 506, the routine module 486 determines whether at least one of theCTPD-DS, CTPD-EV, and CTPD-FR methods is fully activated and generates aroutine output signal ROUT. The CTPD-DS method may be fully activated,for example, when the digital scroll compressor PWM % is at 100%. TheCTPD-EV method may be fully activated when, for example, the MOP-A valueis stored and the overwrite timer 485 is started. The CTPD-FR method maybe fully activated when, for example, the condenser fan speeds have beenreduced as performed at 528. Task 508 is performed when at least one ofthe CTPD-DS, CTPD-EV, and CTPD-FR methods is fully activated based onthe routine output signal ROUT.

At 508, the lag compressor module 345 maintains the lag compressor in anON state for an additional predetermined delay period (e.g., 10 seconds)while the CTPD-DS, CTPD-EV, and CTPD-FR methods become fully activated.This predetermined delay period allows components affected by theCTPD-DS, CTPD-EV, and CTPD-FR methods to perform appropriate adjustmentsbefore the lag compressor is turned OFF. At 509, the lag compressormodule 345 deactivates the lag compressor after the predetermined delayperiod.

At 510, the shutdown parameter module 489 determines the parameterDSPD-A, which is referred to as a digital scroll loading duration. Thisis the amount of time for which the digital scroll compressor operatesat 100% or at the predetermined (or maximum) PWM % after the CTPD-DSmethod becomes active.

At 511, the lead compressor module 487 operates the lead compressor at afully-loaded PWM % equal to 100% or a predetermined PWM %. Thisoverrides a PWM % called for based on the PWMCFC value. The PWM % of thelead compressor may be held at a constant PWM % (100% or thepredetermined PWM %) when the fixed compressor in the tandem set turnsOFF. This ensures that when the fixed scroll compressor turns OFF, thereduction in a volumetric displacement rate of the tandem set is lessthan or equal to 50%. By minimizing the reduction in the volumetricdisplacement rate, the chances of a loss in the compressor pressuredifference are reduced. As a result, the compressor pressure differenceis not reduced.

At 512, the load duration timer 484 is started and generates a loadduration signal TIML. At 514, the lead compressor module 487 determinedwhether the load duration signal TIML has exceeded a predetermined loadduration. The predetermined load duration may be indicated by theparameter DSPD-A. Task 516 is performed when the load duration signalTIML exceeds the predetermined load duration. At 516, the leadcompressor module 487 deactivates the CTPD-DS method and ramps the PWM %of the lead compressor back to the PWMCFC value or a value determinedbased on the PWMCFC value. The CTPD-DS method may end at 532.

In FIG. 21, a digital scroll plot for the CTPD-DS method is shown. FIG.21 includes a digital scroll PWM % signal, a PWMCFC signal, and a lagcompressor status signal. The digital scroll PWM % signal starts at thePWMCFC signal, increases to a peak (e.g., 100%) when the CTPD-DS methodis activated, and then decreases back to the PWMCFC signal when the loadduration time has lapsed. The PWMCFC signal transitions from a low stateto a high state when the lag compressor motor is deactivated.

Also at 517, the shutdown parameter module 489 determines the parametersEVPD-A, EVPD-B, EVPD-C. At 518, the overload module 491 stores theparameter MOP-A in a second location of the memory 186. As a result theMOP-A value is stored in a first location and in a second location ofthe memory 186. At 520, the overwrite module 493 overwrites theparameter MOP-A that is stored in the first location. The overwritemodule 493 stores the parameter EVPD-A as the MOP-A value in the firstlocation. At 521, an EEV opening percentage is controlled based onEVPD-A.

At 522, the overwrite timer 485 is started and generates an overwritetimer signal TIMS. At 524, the expansion valve module 488 determineswhether the overwrite timer signal TIMS has exceeded the parameterEVPD-B. If the parameter EVPD-B has been exceeded, task 526 isperformed.

At 526, the overwrite module 493 changes the parameter MOP-A stored inthe first location back from the parameter EVPD-A to the originalparameter MOP-A stored at the second location. This is done at the ratedefined by parameter EVPD-C. The original parameter MOP-A is copied fromthe second location of the memory 186 to the first location of thememory 186. The CTPD-EV method may end at 532 when the parameter MOP-Ahas been completely restored to its original value.

In FIG. 22, an expansion valve plot for the CTPD-EV method is shown.FIG. 22 includes an EVPID signal, an EEV opening % signal, and a MOP-Asignal, which are associated respectively with the EVPID value, theopening percentage of the EEV, and the parameter MOP-A. FIG. 22 alsoincludes the lag compressor motor status signal. As shown, the LAGcompressor is switched OFF the predetermined period after the parameterMOP-A is set equal to the parameter EVPD-A. At this point, the EEVopening % signal ramps down and remains at a reduced EEV openingpercentage until the overwrite timer is greater than the parameterEVPD-B. The parameter MOP-A is then ramped up to its original value andthe EEV opening % signal is increased to the EVPID value.

The condenser fan module 346 may control the speeds of the condenser fanusing a condenser fan proportional integral derivative (CFPID) method.The speeds of the condenser fan may be controlled to maintain apredetermined head pressure. The CFPID method may be used to generate acondenser output signal CF to control the condenser fan. The CFPIDmethod may be used to generate the condenser output signal CF based on aparameter CF-A. The parameter CF-A refers to a condenser maximum fanspeed, which defines a maximum speed that the condenser fan can operate.The parameter CF-A may be used as a high-limit for the actual speed ofthe condenser fan. As an example, when the condenser output signal CF isat 100%, the parameter CF-A may be set equal to a predeterminedpercentage (e.g., 90%). The condenser fan may operate at thepredetermined percentage until the parameter CFPID is less than thecondenser output signal CF-A.

The CTPD-FR method is used to temporarily reduce the parameter CF-A toprovide a reduction in the speed of the condenser fan. This causes thehead pressure HEAD in the corresponding cooling circuit to increase. Byoperating the tandem set at an elevated head pressure before the lagcompressor turns OFF, loss in the pressure difference can be avoided.

The CTPD-FR method may include use of a parameter FRPD-A. The parameterFRPD-A refers to a condenser fan reduction exponent. When the fixedscroll compressor turns OFF, the parameter FRPD-A is a value that isused to determine a reduction magnitude in a speed of the condenser fanrelative to a change in digital scroll PWM %.

Equation 1 provides a relationship between a maximum allowable condenserfan speed F₁ before the lag compressor is turned OFF and a maximumallowable condenser fan speed F₂ after the lag compressor is turned OFF.

$\begin{matrix}{F_{2} = {F_{1}\left\lbrack \left( \frac{C_{2}}{C_{1}} \right)^{A} \right\rbrack}} & (1)\end{matrix}$

In equation 1, the variable A is equal to the parameter FRPD-A. Theparameter CF-A may be adjusted based on the variables F₁ (original CF-A)and F₂ (modified CF-A when CTPD-FR is started). The variable C₁ is equalto the parameter PWMCFC before lag compressor is turned OFF. Thevariable C₂ is equal to the parameter PWMCFC after the lag compressor isturned OFF. The variable C₂ is calculated by using the call for coolingvalue CFC at the time the CTPD-FR method is started. Based on Equation1, increasing the FRPD-A parameter increases an amount of potentialreduction in the speed of the condenser fan during the CTPD-FR method.

At 528, when the CTPD-FR method becomes active, the condenser fan module346 may evaluate Equation 1. The parameter CF-A or original CF-A valueis copied from a first location of the memory 186 to a second locationof the memory 186. The parameter CF-A is then set equal to the maximumallowable condenser fan speed F₂ by storing F₂ at the first location ofthe memory 186. This causes the condenser fan module 346 to reduce thespeed of the condenser fan to a value equal to the maximum allowablecondenser fan speed F₂. The speed of the condenser fan may be controlledbased on the original CF-A value or the maximum allowable condenser fanspeed F₁ prior to the task 528 and/or prior to the lag compressor beingshut OFF. The changeover to the maximum allowable condenser fan speed F₂may include a step reduction in the speed of the condenser fan when thelag compressor turns OFF, as F₂ may be less than F₁.

At 530, condenser fan module 346 restores the parameter CF-A back to itsoriginal value. The condenser fan is operated based on the CFPID valueto ramp the speed of the condenser fan towards the original CF-A valueand back to a speed prior to the lag compressor being turned OFF. Thisis done at a rate based on proportional (P), integral (I), derivative(D) coefficients associated with the CFPID method. The CTPD methodand/or CTPD-FR method may end at 532.

In FIG. 23, a condenser fan speed plot for the CTPD-FR method is shown.FIG. 23 includes a CF-A signal and a CFPID signal, which correspondrespectively to the parameter CF-A and the CFPID value. FIG. 23 alsoincludes the condenser fan speed signal CF and the lag compressor motorstatus signal. The condenser fan speed signal CF is reduced when theparameter CF-A is reduced at task 528. This may occur at a predeterminedperiod (e.g., 10 seconds) before the lag compressor is turned OFF. Theparameter CF-A is increased back to its original value after the lagcompressor is turned OFF. The condenser fan speed signal CF is ramped upwhen the parameter CF-A is increased, which occurs at task 530.

To prevent and/or mitigate a reduction or a loss in compressor pressuredifference when a compressor is deactivated, the method of FIG. 20manipulates operation of system components in anticipation of one of thecompressors in the tandem set being turned OFF. The method includestasks to prevent occurrence of a low-pressure difference during lagcompressor shutdown. This prevents damage to compressors and helpsdiminish undue reductions in cooling capacity when the lag compressor inthe tandem set of compressors is turned OFF.

The parameters for the method of FIG. 20 may be configurable via theuser interface 329 of FIG. 2. This allows the method to be configurable.The method interrupts operational control of a digital scroll compressorPWM %, an EEV opening position, and speeds of condenser fans whenconditions to turn OFF one of the compressors in the tandem set aresatisfied. This proactively reduces the chances of running the tandemset at low-pressure difference associated with an unloaded compressorcondition.

The method of FIG. 20 may be applied to an air conditioning system with,for example, tandem digital scroll compressors, a micro-channelcondenser, and an electronic expansion valve. As another example, themethod of FIG. 20 may be applied to an air conditioning system withtandem digital scroll compressors and a condenser. The air conditioningsystem may not have an EEV. As yet another example, the method may beapplied to an air conditioning system with tandem digital scrollcompressors and an EEV. The air conditioning system may not have acondenser.

The above-described tasks of the methods of FIGS. 5A-5B, 7A-7B, 11, 13,15, 16, and 20 are meant to be illustrative examples; the tasks may beperformed iteratively, sequentially, synchronously, simultaneously,continuously, during overlapping time periods or in a different orderdepending upon the application. Also, any of the tasks may not beperformed or skipped depending on the implementation and/or sequence ofevents.

The foregoing description is merely illustrative in nature and is in noway intended to limit the disclosure, its application, or uses. Thebroad teachings of the disclosure can be implemented in a variety offorms. Therefore, while this disclosure includes particular examples,the true scope of the disclosure should not be so limited since othermodifications will become apparent upon a study of the drawings, thespecification, and the following claims. For purposes of clarity, thesame reference numbers will be used in the drawings to identify similarelements. As used herein, the phrase at least one of A, B, and C shouldbe construed to mean a logical (A or B or C), using a non-exclusivelogical OR. It should be understood that one or more steps within amethod may be executed in different order (or concurrently) withoutaltering the principles of the present disclosure.

As used herein, the term module may refer to, be part of, or include anApplication Specific Integrated Circuit (ASIC); an electronic circuit; acombinational logic circuit; a field programmable gate array (FPGA); aprocessor (shared, dedicated, or group) that executes code; othersuitable hardware components that provide the described functionality;or a combination of some or all of the above, such as in asystem-on-chip. The term module may include memory (shared, dedicated,or group) that stores code executed by the processor.

The term code, as used above, may include software, firmware, and/ormicrocode, and may refer to programs, methods, functions, classes,and/or objects. The term shared, as used above, means that some or allcode from multiple modules may be executed using a single (shared)processor. In addition, some or all code from multiple modules may bestored by a single (shared) memory. The term group, as used above, meansthat some or all code from a single module may be executed using a groupof processors. In addition, some or all code from a single module may bestored using a group of memories.

The apparatuses and methods described herein may be implemented by oneor more computer programs executed by one or more processors. Thecomputer programs include processor-executable instructions that arestored on a non-transitory tangible computer readable medium. Thecomputer programs may also include stored data. Non-limiting examples ofthe non-transitory tangible computer readable medium are nonvolatilememory, magnetic storage, and optical storage.

Although the terms first, second, third, etc. may be used herein todescribe various elements, components and/or modules, these items shouldnot be limited by these terms. These terms may be only used todistinguish one item from another item. Terms such as “first,” “second,”and other numerical terms when used herein do not imply a sequence ororder unless clearly indicated by the context. Thus, a first itemdiscussed herein could be termed a second item without departing fromthe teachings of the example implementations.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A method comprising: determining whether an OFFcriterion is satisfied for a lag compressor of a tandem set of scrollcompressors, wherein the tandem set of scroll compressors comprises alead compressor; initiating at least one process when the OFF criterionis satisfied; and maintaining the lag compressor in an ON state for apredetermined period subsequent to the OFF criterion being satisfied,wherein the at least one process comprises at least one of operating thelead compressor at a maximum level, overriding a motor overloadprotection method, wherein the motor overload protection method protectsmotors of the lead compressor and the lag compressor, and overriding aproportional integral derivative (PID) method to reduce a speed of acondenser fan, wherein the PID method controls the speed of thecondenser fan.
 2. The method of claim 1, wherein the maximum level isequal to 100%.
 3. The method of claim 1, wherein: the at least oneprocess comprises a scroll method; and the scroll method comprisesdetermining a loading duration of the lead compressor, operating thelead compressor at the maximum level, starting a load duration timer andgenerating a timer signal, and deactivating the scroll method andramping the lead compressor to a predetermined value when the timersignal is greater than the loading duration.
 4. The method of claim 3,further comprising: determining a cooling value; and determining thepredetermined value based on the cooling value, wherein thepredetermined value is a pulse width modulation percentage for the leadcompressor.
 5. The method of claim 4, further comprising: determining atemperature error value; and determining a number of the compressors inthe tandem set that are activated, wherein the cooling value isdetermined based on the temperature error value and the number of thecompressors that are activated.
 6. The method of claim 1, wherein the atleast one process comprises: copying an overload temperature thresholdused by the motor overload protection method from a first memorylocation to a second memory location; overwriting the overloadtemperature threshold stored in the first memory location with apredetermined value; controlling an opening percentage of an electronicexpansion valve based on the predetermined value; starting an overwritetimer and generating a timer signal; comparing the timer signal to anexpansion valve duration; and changing the predetermined value at thefirst memory location back to the overload temperature threshold whenthe timer signal is greater than the expansion valve duration.
 7. Themethod of claim 6, wherein the changing of the predetermined value atthe first memory location back to the overload temperature thresholdcomprises ramping the predetermined value to the overload temperaturethreshold at a predetermined rate.
 8. The method of claim 6, furthercomprising: maintaining the opening percentage of the electronicexpansion valve at a predetermined percentage based on a proportionalintegral derivative (PID) method prior to initiating the at least oneprocess; decreasing the opening percentage subsequent to initiating theat least one process; and increasing the opening percentage to thepredetermined percentage when when the timer signal is greater than theexpansion valve duration.
 9. The method of claim 1, further comprising:controlling a speed of the condenser fan based on a first maximum speedprior to turning OFF the lag compressor; and controlling the speed ofthe condenser fan based on a second maximum speed after turning OFF thelag compressor, wherein the at least one process comprises determiningthe second maximum speed based on the first maximum speed, a firstpredetermined value, a second predetermined value, and a condenser fanreduction exponent.
 10. The method of claim 9, wherein the at least oneprocess further comprises ramping up the speed of the condenser fan (i)subsequent to controlling the speed of the condenser fan based on thesecond maximum speed, and (ii) based on the first maximum speed.
 11. Themethod of claim 9, further comprising: determining a cooling value;determining the first predetermined value based on the cooling value,wherein the first predetermined value is a first pulse width modulationpercentage for the lead compressor before the lag compressor is turnedOFF; and determining the second predetermined value based on the coolingvalue, wherein the second predetermined value is a second pulse widthmodulation percentage for the lead compressor after the lag compressoris turned OFF.
 12. The method of claim 11, further comprising:determining a temperature error value; and determining a number of thecompressors in the tandem set that are activated, wherein the coolingvalue is determined based on the temperature error value and the numberof the compressors that are activated.
 13. The method of claim 1,further comprising: controlling a speed of the condenser fan based on(i) the PID method, and (ii) a first maximum speed prior to turning OFFthe lag compressor, wherein the speed of the condenser fan is at anoriginal speed prior to the lag compressor being turned OFF; reducingthe first maximum speed to a second maximum speed after turning OFF thelag compressor; adjusting the speed of the condenser fan to the secondmaximum speed; and ramping up the speed of the condenser fan from thesecond maximum speed to the original speed.
 14. The method of claim 1,wherein the at least one process comprises: operating the leadcompressor at the maximum level; overriding the motor overloadprotection method; and overriding the PID method.
 15. A systemcomprising: a lag compressor module configured to determine whether anOFF criterion is satisfied for a lag compressor of a tandem set ofscroll compressors, wherein the tandem set of scroll compressorscomprises a lead compressor; and a lead compressor module configured to,if the OFF criterion is satisfied, at least one of, operate the leadcompressor at a maximum level, override a motor overload protectionmethod, wherein the motor overload protection method protects motors ofthe lead compressor and the lag compressor, and override a proportionalintegral derivative (PID) method to reduce a speed of a condenser fan,wherein the PID method controls the speed of the condenser fan, whereinthe lag compressor module is configured to maintain the lag compressorin an ON state for a predetermined period subsequent to the OFFcriterion being satisfied.
 16. The system of claim 15, furthercomprising: a load duration timer configured to determine a loadingduration of the lead compressor; and an overwrite timer configured togenerate a timing signal, wherein the lead compressor module isconfigured to, if the OFF criterion is satisfied, perform a scrollmethod including determining a loading duration of the lead compressorvia the load duration timer, operating the lead compressor at themaximum level, and deactivating the scroll method and ramping the leadcompressor to a predetermined value when the timing signal is greaterthan the loading duration.
 17. The system of claim 15, furthercomprising an expansion valve module configured to, if the OFF criterionis satisfied: copy an overload temperature threshold used by the motoroverload protection method from a first memory location to a secondmemory location; overwrite the overload temperature threshold stored inthe first memory location with a predetermined value; control an openingpercentage of an electronic expansion valve based on the predeterminedvalue; start an overwrite timer and generating a timer signal; comparethe timer signal to an expansion valve duration; and change thepredetermined value at the first memory location back to the overloadtemperature threshold when the timer signal is greater than theexpansion valve duration.
 18. The system of claim 15, furthercomprising: a condenser fan module configured to control (i) a speed ofthe condenser fan based on a first maximum speed prior to turning OFFthe lag compressor, and (ii) the speed of the condenser fan based on asecond maximum speed after turning OFF the lag compressor; and a controlmodule configure to, if the OFF criterion is satisfied, determine thesecond maximum speed based on the first maximum speed, a firstpredetermined value, a second predetermined value, and a condenser fanreduction exponent.
 19. The system of claim 15, further comprising acondenser fan module configured to: control a speed of the condenser fanbased on (i) the PID method, and (ii) a first maximum speed prior toturning OFF the lag compressor, wherein the speed of the condenser fanis at an original speed prior to the lag compressor being turned OFF;reduce the first maximum speed to a second maximum speed after turningOFF the lag compressor; adjust the speed of the condenser fan to thesecond maximum speed; and ramping up the speed of the condenser fan fromthe second maximum speed to the original speed.
 20. The system of claim15, wherein the lead compressor module is configured to: operate thelead compressor at the maximum level; override the motor overloadprotection method; and override the PID method.